1. Field of the Invention
This invention relates to an improved temperature sensor for detecting an abnormally or dangerously high temperature of a substrate. More particularly, the invention concerns a temperature sensor for monitoring the condition of bearings, motors, and other devices which increase in temperature as they begin to fail.
2. General Discussion of the Background
The temperature of a machine part is often an important indication of its mechanical condition. Railroad cars, for example, have journal bearings with a normal operating temperature of about 40.degree. C. above the ambient temperature. If the bearing temperature rises to between 120.degree. and 180.degree. C. above ambient, there will almost always be a complete bearing failure. Similar dramatic rises in temperature, which are well in excess of temperature spikes encountered during ordinary operation, can be found in truck axle bearings, factory machine bearings, motors, and other devices which are about to fail. If the problem which is inducing failure is addressed before failure occurs, a total breakdown can often be avoided. Unfortunately, the problem is seldom recognized before failure occurs because a temperature increase is not often visually apparent.
Signaling devices which provide a visual indication of temperature increases in bearings have previously been proposed. For example, U.S. Pat. Nos. 3,339,518 and 4,501,006 disclose indicators for detecting overheated bearings in vehicles such as railroad cars. Both of these devices include rigid tubular metal elements which are filled with a signal substance and closed with a plug. As the temperature inside the rigid container increases, its internal pressure correspondingly rises. When the differential pressure across the plug reaches a preselected value, it forces the plug out of the container to allow release of the signal substance into the environment. The signal substance, which is usually a colored dye, can be visually detected and remedial mechanical repairs made before ultimate failure occurs. Although such prior art signaling devices provide advance warning of mechanical failure, their operation depends on establishment of a differential pressure across the plug. This is a problem because the differential pressure can be affected by changing altitude of the railroad car. Another drawback of these devices is that they are rigid and cannot conform to the surface contours of a variety of objects having different shapes.
U.S. Pat. Nos. 3,569,695; 3,877,411; and 4,459,046 all show temperature signaling devices which incorporate an indicator wafer that changes color when its surrounding temperature exceeds a safe level. Although these devices are not affected by changing altitude, they still suffer from the problem of being flat and rigid. They are therefore useful primarily on flat surfaces and cannot be flexed to conform to the shape of a nonplanar substrate.
U.S. Pat. No. 2,694,997 discloses a bearing failure indicator which includes a colored wafer that melts upon reaching a certain temperature. U.S. Pat. No. 4,265,117 shows a thermocouple which is attached to a flexible, disc-like metal plate which has sufficient flexibility to conform to the general shape of a surface.
It is an object of this invention to provide a temperature signal indicator which can easily be adapted for use with a variety of objects having different shapes.
It is yet another object of the invention to provide such an indicator that can conform to the shape of a nonplanar surface of an object.
Yet another object of the invention is to provide an indicator which is not affected by changing altitude.